Three AI coding agents leaked secrets through a single prompt injection. One vendor's system card predicted it

A security researcher, working with colleagues at Johns Hopkins University, opened a GitHub pull request, typed a malicious instruction into the PR title, and watched Anthropic’s Claude Code Security Review action post its own API key as a comment. The same prompt injection worked on Google’s Gemini CLI Action and GitHub’s Copilot Agent (Microsoft). No external infrastructure required.

Aonan Guan, the researcher who discovered the vulnerability, alongside Johns Hopkins colleagues Zhengyu Liu and Gavin Zhong, published the full technical disclosure last week, calling it “Comment and Control.” GitHub Actions does not expose secrets to fork pull requests by default when using the pull_request trigger, but workflows using pull_request_target, which most AI agent integrations require for secret access, do inject secrets into the runner environment. This limits the practical attack surface but does not eliminate it: collaborators, comment fields, and any repo using pull_request_target with an AI coding agent are exposed.

Per Guan’s disclosure timeline: Anthropic classified it as CVSS 9.4 Critical ($100 bounty), Google paid a $1,337 bounty, and GitHub awarded $500 through the Copilot Bounty Program. The $100 amount is notably low relative to the CVSS 9.4 rating; Anthropic’s HackerOne program scopes agent-tooling findings separately from model-safety vulnerabilities. All three patched quietly, and none had issued CVEs in the NVD or published security advisories through GitHub Security Advisories as of Saturday.

Comment and Control exploited a prompt injection vulnerability in Claude Code Security Review, a specific GitHub Action feature that Anthropic’s own system card acknowledged is “not hardened against prompt injection.” The feature is designed to process trusted first-party inputs by default; users who opt into processing untrusted external PRs and issues accept additional risk and are responsible for restricting agent permissions. Anthropic updated its documentation to clarify this operating model after the disclosure. The same class of attack operates beneath OpenAI’s safeguard layer at the agent runtime, based on what their system card does not document — not a demonstrated exploit. The exploit is the proof case, but the story is what the three system cards reveal about the gap between what vendors document and what they protect.

OpenAI and Google did not respond for comment by publication time.

“At the action boundary, not the model boundary,” Merritt Baer, CSO at Enkrypt AI and former Deputy CISO at AWS, told VentureBeat when asked where protection actually needs to sit. “The runtime is the blast radius.”